Bug 36758: Add trigger for assignee notification trigger

Bug 36758: Add trigger for assignee notification trigger

This patch adds a new notice trigger to allow notifying assigned staff
that they have been assigned a new catalog concern to action.

Test plan
1) Enable catalog concerns (either via staff or opac interfaces)
2) Report a new concern (either via staff or opac interfaces)
3) As a staff user, assign the concern to another staff user
4) Check that the new 'TICKET_ASSIGNED' notice has be queued for that
   staff user.
5) Re-assign the catalog concern to yourself
6) Note that you do not see a 'TICKET_ASSIGNED@ notice queued as you
   self-assigned.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>